Electric coupling circuits



May 23, 1933. H. A. WHEELER 1,910,870

ELECTRIC COUPLING CIRCUITS Filed Sept. 10, 1930 Patented May 23, 1933 UNITED STATES,

PATENT OFFICE HAROLD A. WHEELER, OF GREAT NECK, NEW YORK, ASSIGNOR T0 HAZELTINE COB- PORATION, A CORPORATION 01" DELAWARE ELECTRIC COUPLING CIRCUITS Application filed September 10, 1930. Serial No. 480,949.

This invention relates to high frequency electric coupling circuits tunable over a range in frequencyand adapted more especially for interconnecting successive elements of a high frequency thermionic amplifier or a radio system.

The coupling circuits of the present invention are characterized in that the voltage amplification produced thereby iscaused to vary automatically with the frequency of tuning in a manner which is under the control of the designer; the design in general being preferably such as to produce a substantially constant resultant amplification, i. e., uniform sensitivity throughout the tunable frequency range.

With an elementary type of high frequency coupling circuit of which a two-winding transformer havig a tunable secondary is a typical example, the variation of amplification with frequency is not under the control of the designer, but increases with frequency throughout the tunable range in a manner well understood, with consequent detrimental effects on the amplification, selectivity and stability of the system.

In the elementary type of coupling circuit referred to above there exists between the primary and tunable secondary circuits a coefiicient of coupling which is substantially constant throughout the frequency range, a condition which is partially responsible for the mentioned increase in amplification with frequency. The present invention avoids the above noted objections by proper cooperation of different reactance elements in the "in but differing from the present invention primary circuit. These provide an effective coupling between input and tunable output as regards both the circuit connections utilized and the specific modes of operation thereof, are set forth in Patent No. 1,7 63,380 issued to C. E. Trube, and in his copending application Serial No. 101,906 filed April 14, 1926 and Serial No. 120,045 filed July 2, 1926.

The coupling circuits of the present invention embody the following essential elements, namely: a tunable secondary circuit including a fixed inductive winding and a variable capacity bridged between output terminals, and a primary circuit including in closed series connection in the order recited, a fixed capacity, a fixed inductance magnetically uncoupled to the secondary winding, a primary winding magnetically coupled to said secondary winding, an input terminal connected between the fixed capacity and fixed inductance, and a second input terminal connected to an intermediate point of the primary windmg. The mentioned connection of the input terminals to the closed primary circuit provides a pair of parallel paths therebetween, one path containing the fixed capacity and a portion of the primary winding, w ile the other path contains the fixed uncoupled inductance and the remaining portion of the primary winding.

An additional capacity may or may not be connected between the input terminals, or in the operation of the circuit, capacity may exist between these terminals, which is part of the input circuit connected thereto. Irrespective of the particular arrangement utilized, the resultant capacity associated with the inductance of the primary circuit, including the fixed capacity of the closed series connection as well as any capacity existing between the input terminals in the operation of the circuit, should be such as to make the primary circuit resonant at a frequency lower, but not greatly lower, than the lowest frequency of the tunable range.

The objects obtained by such design (1) provide an efiective coupling between primary and secondary circuits which automatically falls off as the frequency of tuning is increased but at a rate proportioned to produce a desired ariation in amplification with frequency; and (2) control the variations of the input impedance of the coupling circuit throughout the tunable range in such manner as to insure in operation, improved stability,

. nals I lncludes in closed series connection inselectivity and sensitivity in the amplifier stages. a

The manner in which the results of the present invention are achieved will be more clearly understood upon detailed conslderation of the circuit connections set forth in the drawing of which:

Fig. 1 shows in diagrammatic form the elementary coupling circuit of the present mvention;

Figure 2 shows the coupling circuit of Fig. 1 as utilized for connectmg an antenna 011'- cuit to the in ut of a thermionic valve;

Fig. 3 is a iagrammatic representation of the coupling circuit as employed for interconnectm a pair of screen-grid tubes in cascade re ation;

Fig. 4 shows an intersta'ge connection employing triode tubes wherein the coupllng circuit is arranged in conjunction with a suitable capacity to provide neutralization of the interelectrode tube capacities, while Fig. 5 shows a modified form of the invention employedas an interstage coupling c1rcuit.

Corresponding elements are similarly designated in the several figures.

Referring to Fig. 1 of the drawing the coupling circuit comprises a tunable secondary circui consisting of a fixed inductive windin L and a variable tuning condenser C bridge between output terminals 0. The primar circuit associated with input termithe order recited, a fixed capacity G a fixed inductance L magnetically uncoupled to the secondary winding L, and a primary winding comprising the portions L and L ma netically coupledtothe secondary winding The upper input terminal is connected be-,

tween the elements C and L while the lower input terminal is tapped to an intermediate point of the primary winding, as indicated.

The capacity C shown dotted, is intended to represent the entire capacity effectively brid ed between the input terminals I, while the c1rcuit is in operation, and any additional capacities associated with the primary circuit inductance, and in conjunction with capacity C, should be such as to render the primary circuit resonant at a frequency sli htly below the tunable range of the SGCOIMELI'y circuit LC. It is not essential to the operation of the circuit that a capacity C exist between the' input terminals I. In the event that the capacity C is zero, however, the capacity C should be of such magnitude as to render the primary circuit resonant at the low frequency above specified.

The elements of the primary circuit as thus proportioned assure that the path between the input terminals I containing the elements C and L will be predominantly capacitively reactive throughout the tunable range and hence of decreasing impedance for increases in tuned frequency; whereas the path containing elements L and L is inductively reactive and its impedance increases in magnitude with the tuned frequency. It will thus be apparent that the winding portions L and L produce effectsin the secondary circuit which vary in opposite fashion with the tuning adjustment. The result 1s that there is produced an efi'ective coupling between primary and secondary circuits which automatically decreases as the frequency of tuning increases, in a desired manner.

In Fig. 2 the input terminals I of the coupling circuit are connected to an antenna 1 and a ground 2 respectively; while the output terminals 0 are connected to the grid and cathode respectively of a thermionic tube V The capacity C of Fig. 1 comprises in Fig. 2 the efiective antenna-to-ground capacity of the antenna circuit 1, which in conjunction with the capacity C renders the primary circuit inductance resonant at a frequency below the tunable range.

In Fig. 3 wherein the coupling circuit interconnects a pair of thermionic tubes in cascade relation, the input terminals I are connected to anode and cathode respectively of the first tube V while the output terminals are connected to grid and cathode respectively of a second tube V Space current for the tube V is supplied from a battery B inserted in the conductor 3 extending from the point between windings L and L to the grounded conductor 4. The capacity C of Fig. 1 comprises in Fig. 3 the anode-toground capacity of tube V which in conjunction with the capacity C renders the primary circuit inductance resonant at the low frequency above specified.

The coupling circuit of Fig. 4 interconnects a pair of triode tubes in cascade relation, each tube having anode, cathode and grid electrodes only. "In order to neutralize the gridtoanode capacity C of tube V a condenser C is connected from the anode thereof to the lower terminal of winding L the latter terminal being of opposite polarity to the upper terminal of win ing L, thereby providing a circuit adapted to accomplish this neutralization. The neutralization is accomplished in the manner disclosed in Patent No. 1,489,228 lssued to L. A. Hazeltine April 1, 1924. In

-order to accomplish satisfactory neutralizain cascade relation, which is a slight modification of the circuit of Fig. 1 in that the connection of the input terminals of the coupling circuit are reversed and the location of the B 5 battery in the circuit is changed as required. The circuit of Fig. 5 has certain structural advantages as compared to Fig. 1. For example, with the arrangement of Fig. 5, by winding the coils L and L about the lower 10 portion of the secondary winding L, it is possible to dispense with a physical condenser (1 since the natural capacity existing between the primary windings L and L, and secondary winding L may be utilized for this purpose, leaving the lower terminal of winding L on open circuit.

I claim: '1. An electric coupling circuit tunable throughout a range in frequency comprising a secondary circuit including a secondary winding and variable capacity bridged between output terminals, and a primary circuit including in closed series connection, a second capacity, an inductance winding noninductively coupled to said secondary, and a primary winding inductively coupled to said secondary, an input terminal connected between said second capacity and inductance winding, and a second input terminal connected to an intermediate point of said rimary winding, said primary winding an inductance winding in conjunction with associated capacities having a natural frequency slightly below said tunable range.

2. A tuned radio-frequency coupling system comprising a variable tuning condenser, a secondary coil connected in parallel therewith, a primary coil inductively coupled to said secondary coil, an input terminal connected to a tap intermediate the ends of said primary coil, a second input terminal, a fixed capacity effectively connected between said second terminal and one end of said primary coil and a fixed inductance winding non-inductively coupled to said seconda connected between said second terminal an the other end of said primary coil, said primary coil together with said fixed inductance windin and associated capacities havin a nature frequency slightl below the tuning range.

3. A tuned ra io-frequency couplin system comprising a variable tuning con enser, a secondary coil connected in parallel therewith, a primary coil inductively coupled to said secondary coil, an input terminal connected to a tap intermediate the ends of said primary coil, a second input terminal connected to one end of said primary coil throu h an inductance winding, no connection to e other end of said primary coil, and said rimary and secondary coils bein arrang to provide natural capacity there tween, said primary coil toget er with said inductance and associated capacity having'a natural frequency slightly below the tuning range.

4. In combination a thermionic tube having anode, cathode and grid, a tuned radio frequency coupling system including a secondary transformer coil and'a varlable capacity bridged between said cathode and r1d, a primary coil inductively related to said secondary, means connecting a first input terminal to an intermediate point of said prlmary coil and to said cathode, means connecting a second input terminal through a fixed capacity to the end of said primary coil which is of like alternating current polarity to the grid terminal of said secondary coil, a fixed inductance winding connected from the other end of said primary to said second terminal, sa1d fixed inductance winding and primary coil together with the associated capacity in the operation of said system being resonant at a frequency slightly below the tuning range of said variable condenser and acondenser connected from said anode to a point between said fixed inductance winding and primary coil, said condenser being proportioned to neutralize at least in part the interelectrode capacitive coupling of said tube.

In testimony whereof I aifix my si ature.

HAROLD A. WHEE ER. 

